Turbofan gas turbine engines employ high and low pressure turbines that drive a high pressure compressor and low pressure compressor, respectively. The turbines and their compressors are interconnected by a concentric pair of shafts, the outer shaft typically interconnecting the high pressure turbine and compressor while the inner shaft interconnects the low pressure turbine and compressor. During engine operation, the two shafts both rotate but at different speeds relative to each other, necessitating shaft bearings therebetween. It is also necessary to confine the liquid lubricant used for the shaft bearings to a volume immediately surrounding the bearings, and at the same time prevent excessive amounts of cooling air to flow into that volume of lubricating liquid.
Due to the high temperatures and relatively high rotational speeds of the shafts, often exceeding thousands of revolutions per minute, conventional contacting seals between the shafts are inappropriate. Consequently, labyrinth seals, which comprise a plurality of array of radially-projecting seal teeth, are used between the two shafts. Typically, the teeth are mounted on or are integral with one of the shafts and project toward the other shaft, such that the teeth and an area of the other shaft swept by the teeth form a rotating air seal. The teeth and shafts are manufactured and assembled to minimize the radial gap therebetween and thereby promote the effectiveness of the air seal. As a result, some degree of rubbing between the teeth and the area swept by the teeth occurs during the initial operation of the engine due to manufacturing tolerances, differing rates of thermal expansion and dynamic effects. However, direct rubbing contact between the teeth and the shaft tends to abrade the teeth, which further increases the radial air seal gap and shortens the useful life of the labyrinth seal. As such, it is well known in the art to form an abrasive aluminum oxide (alumina) coating on the teeth and an abradable material on the area of the shaft swept by the teeth. The alumina coating is typically adhered to the teeth with a bond coat, typically a nickel-aluminum alloy. Abrasive alumina coatings have found wide use as being capable of surviving in the hostile environment of a gas turbine engine, and therefore able to protect the teeth during numerous rub encounters that occur during in-service operation of the engine.
In addition to being subject to wear from rub encounters, alumina seal teeth coatings are susceptible to damage from erosion and attack from environmental contaminants. Consequently, replacement of the coating is eventually required. In addition, the alumina coating must also be removed on occasion to permit inspection of the underlying teeth, which can crack, oxidize and erode as a result of the hostile operating environment of a gas turbine engine. Because alumina is extremely chemically resistant and cannot be removed using conventional stripping chemicals at room temperature and pressure, the repair of alumina seal teeth coatings has generally required removal by mechanical means, such as abrasive blasting or by waterjet. Thereafter, the underlying bond coat is removed using known etchants.
A disadvantage with this process for removing alumina coatings is that the geometry of the teeth is such that blasting is difficult and coating removal is frequently incomplete. Incomplete removal of the coating prevents complete removal of the underlying bond coat, compromising replacement of the bond coat and inspection of the teeth. Furthermore, if abrasive blasting is used, abrasive grit entrapped in the surfaces of the teeth can cause a reduction in the fatigue life of the teeth and the air seal. Consequently, the teeth must be carefully masked to ensure that only the alumina coating is subjected to the force of the abrasive grit blast. However, masking must be precisely performed to achieve suitable results, and some seal geometries significantly increase the difficulty, and therefore the cost, of removing the abrasive alumina coating.
Thus, it would be desirable to provide a process for removing an abrasive coating, and particularly an abrasive alumina labyrinth seal coating, that achieves removal of the coating without damage to the underlying substrate, is relatively rapid and compatible with turbine component processing methods, and can be used regardless of substrate geometry.